class Solution {
    int dx[4] = {0, 0, -1, 1};
    int dy[4] = {1, -1, 0, 0};
public:
    vector<vector<int>> updateMatrix(vector<vector<int>>& mat) {
        int m = mat.size(), n = mat[0].size();
        vector<vector<int>> dist(m, vector<int>(n, -1));
        queue<pair<int, int>> q;
        for(int i = 0; i < m; i++) // 把所有的源点加⼊到队列中
        {
             for(int j = 0; j < n; j++)
             {
                if(mat[i][j] == 0)
                {
                    q.push({i, j});
                    dist[i][j] = 0;
                }
            }
        }
        while(!q.empty()) // ⼀层⼀层往外扩
        {
            auto [a, b] = q.front();
            q.pop();
            for(int i = 0; i < 4; ++i)
            {
                int x = a + dx[i], y = b + dy[i];
                if(x >= 0 && x < m && y >= 0 && y < n && dist[x][y] == -1)
                {
                    dist[x][y] = dist[a][b] + 1;
                    q.push({x, y});
                }
            }
        }
        return dist;
    }
};